In recent years, there are rapidly growing demands for small-size lenses with widespread use of digital cameras. As an optical element material for producing such small-size lenses, an ultrahigh-refractivity glass having a refractive index (nd) of 2 or more is suitable. As the above ultrahigh-refractivity glass, for example, a translucent ceramic material is known (for example, see JP-A-2004-18286).
However, while the above translucent ceramic has ultrahigh refractivity performances, it is fragile and has problems in processability and shapeability. Further, there is sometimes caused a problem that fine gas bubbles exist in grain boundaries and scatter light.
Therefore, if high refractivity can be realized with a glass, it is expected that the above problems of the ceramic can be overcome. For realizing ultrahigh refractivity performances with a glass, however, a large amount of PbO that has an adverse environmental effect is required, and it is difficult to provide a glass excellent in stability without using a large amount of PbO.